The present invention is generally directed to machine-readable symbols. More particularly, this invention is directed to an identification symbol that can be used on items to be identified and more particularly, to a symbol that includes an orientation finder pattern integrated with data cells and a method for recognizing and decoding the information represented by the symbol.
Machine-readable symbols are well known and commonly used to identify and track products through manufacturing, storage, distribution, and in retail transactions. Probably the best-known example is the use of bar codes to automate checkout and inventory tracking in the grocery store. Traditional bar codes encode characters or digits as patterns of parallel bars of various widths in a single row. Such bar codes, known as “linear bar codes” are scanned and decoded by examining the reflections of the bars along a line roughly perpendicular to the bars of the symbol. The overall width of linear bar code symbols is roughly proportional to the number of characters or digits encoded. However, the linear relationship between the width of the bar code symbol and the data it contains limits the practical data capacity of linear bar code symbols.
Several machine-readable symbols have been introduced which overcome the limited capacity of linear bar codes by representing the encoded data in a two-dimensional matrix pattern. Generally, these two-dimensional symbols graphically represent a binary encoding of the data in contrasting colors—one color for a bit value of “1” and a contrasting color for a bit value of “0”. Such “two-dimensional” symbols generally have much higher data densities and capacities than linear bar codes.
In addition to the encoded data, the symbols typically have other graphic indicia called a “finder pattern” which is used to determine the position and orientation of the data cells. A good finder pattern has characteristics that allow the position, orientation, and scale of the data cells to be determined accurately with a minimum amount of computation. However, the finder patterns in prior art symbols add spatial overhead and reduce data density. Although a finder pattern adds spatial overhead, symbols that lack a finder pattern, such as those symbols disclosed in U.S. Pat. No. 5,946,414 to Cass et al., rely on calibration and alignment of the marking device and the reader to determine orientation and scale. Essentially, when lacking a finder pattern, an exhaustive search of the image by the reader must be conducted to determine proper position and orientation. Thus, it will be appreciated by those skilled in the art that both the traditional practice of adding a finder pattern and the omission of the finder pattern present drawbacks.
For several reasons, at the time of decoding, the cells within the symbol matrix may not accurately represent the intended binary pattern. For example, if the symbol is scratched or partly covered with foreign matter the “light cells” may appear to be “dark cells” or vice versa. To allow accurate recovery of the original information when damage occurs to the data cells, known symbols typically include additional information such as Reed-Solomon Error Detection And Correction (EDAC) code words. Inclusion of such EDAC information allows the recovery of the original information so long as the amount of damage does not exceed the error correcting capacity of the EDAC technique employed. However, symbol damage occurring in the finder pattern is not corrected and may render the symbol unreadable (even if there is no damage to the data areas of the symbol) if the damage interferes with the characteristic used to find the symbol. Finder patterns that rely on the detection of small details are more vulnerable to damage because even a small amount of damage may obliterate the detail. Symbols with larger finder patterns can tolerate more finder pattern damage but in all known symbols this increased tolerance comes with the price of lower data density.
Accordingly, there is a continuing need for a machine readable symbol design that allows increased data density and improved tolerance to damage in both the data and finder patterns. There is also a need for a symbol design which increases the error detection and correction capability by providing additional means of recognizing the location of damage. What is further needed is the increase of efficiency and reliability of the finder pattern. Also, means to encode a plurality of messages within the same graphic symbol are needed. The present invention fulfills these needs and provides other related advantages.